(a) Field of the Invention
This invention relates to acrylic fibers with high physical properties suitable as material for cement reinforcement, and to a method of industrial production of the same.
(b) Description of the Prior Art
Fibers for which high strength and high modulus of elasticity are required have a wide field of use, with the use as reinforcement material being of central importance.
In recent years, as for acrylic fibers, active attempts have been made at the improvement of the physical properties.
As one of such attempts, a method may be mentioned, as described in Japanese Patent Kokai (Laid-open) No. 134,124/79, wherein fibers produced in the usual way are re-stretched in pressurized steam, thereby attaining high stretching times and finally producing fibers with high physical properties.
Another attempt, as represented by Japanese Patent Kokai (Laid-open) No. 199,809/84, is to attain high physical properties by using a superhigh molecular weight polymer as the fiber-forming starting material; arranging polymer molecular chains in parallel in the steps of preparing the spinning solution, spinning the solution into fibers, etc. by various contrivances, thereby bringing the whole molecular chains near to the so-called "stretched-to-full-length" state in the fiber axis direction.
In the above-mentioned re-stretching method in pressurized steam, there are problems in apparatus (such as sealing) and operation, resulting from the use of pressurized steam. This method makes it possible to attain high stretching times by lowering the cohesive force of nitrile groups by the so-called plasticizing effect of water, but on the other hand, it is difficult for this method to exert the original stretching effect of bringing the whole molecular chains near to the "stretched-to-full-length" state, because molecular slipping may occur. For this reason, extremely high stretching times, preferably 35 to 100 times, are necessary as described in the above-mentioned Japanese Patent Kokai (Laid-open) No. 134,124/49. Moreover, such a stretching method in steam is liable to produce microvoids in the fiber structure, and these microvoids obstruct the attainment of high physical properties.
Furthermore, in the method using a superhigh molecular weight polymer, it is necessary to prepare a special polymer which is different from polymers of general use, and with the increase of the degree of polymerization, the viscosity of the polymer solution (spinning solution) increases remarkably, so that the handling, defoaming, spinning, etc. of the solution become difficult. When the polymer concentration of the solution is lowered to lower the viscosity, the productivity and the physical properties of the fibers obtained become low.
Therefore, the object of this invention is to provide acrylic fibers with high physical properties suitable as material for cement reinforcement and a method of producing the same, without the above-mentioned problems.